1. Field of the Invention
This invention is concerned with superplastic forming and in particular with an improved method of superplastic forming sandwich structures from a plurality of aluminum alloy sheets.
2. Discussion of Prior Art
The formation of sheet metal into useful configurations is usually achieved by forces applied to the material by hard tooling, such as in press forming. By contrast, in superplastic forming processes, the required deformation of the workpiece is accomplished by application of gas pressure. A die cavity delimits expansion of the workpiece, but intimate contact between the die and the deforming sheet does not occur until the desired configuration has been achieved in the vicinity of the contact zone.
Many aluminum alloys have the potential to undergo superplastic forming and consequently there is a desire amongst aerospace manufacturers to produce aluminum alloy structures using combined diffusion bonding/superplastic forming processes (DB/SPF) similar to those currently used for titanium components.
However, the propensity of titanium alloys to undergo diffusion bonding is superior to that of the majority of structural aluminum alloys and in many cases strengths of titanium diffusion bonds are comparable to that of the bulk metal. By contrast, the bond interface in joints between aluminum alloys may be much weaker than the parent material. This is especially true at the elevated temperatures required for superplastic forming. As a result of this, when a stress is applied normal to the diffusion bond, the joint is susceptible to peeling under a relatively low peel force.
Low peel strength precludes or limits the use of diffusion bonds in multiple sheet structures with the result that manufacturers are actively seeking alternatives to superplastic forming of aluminum structures lest peel fracture should occur before stresses in the sheet assembly reach the levels required to effect superplastic deformation.
In our earlier patent application GB 2 241 914 A we disclose a method for diffusion bonding aluminum-lithium alloys which gives rise to materials having improved properties compared with previously-known methods. The resultant materials are capable of withstanding high shear stresses, but they have insufficient peel strengths to undergo superplastic forming except in the case of very thin sheet sections of 1 mm thickness or less.
This is in marked contrast to the high peel strengths observed in diffusion bonded titanium alloy joints and it is therefore an object of the present invention to effect an increase in the peel resistance of diffusion bonded aluminum alloys to the extent that such materials are rendered suitable for processing by superplastic forming.